In recent years, with an increasing demand of color copying in an office, a high-speed and downsized full-color copying machine is required, and a method of using a vertical stirring developing device in which two developing screws are vertically placed has been proposed. In such a vertical stirring developing device, a developer is circulated between the vertically placed two screws. However, power of supplying the developer from a lower screw toward an upper screw largely depends on driving stops of the screws, and therefore, with respect to the image forming apparatus having particularly a plurality of image forming stops, a developing sleeve and the two screws are provided with separate driving sources, respectively, in many cases (Japanese Laid-Open Patent Application 2010-112973).